The purpose of this project is to produce infectious RSV from cDNA. This would make it possible to produce and characterize attenuated strains by direct genetic engineering. To date, we have successfully "rescued" a cDNA-encoded genome analog which is 49.3% of full-length and includes a foreign reporter gene under the control of RSV transcriptive signals. Standard infectious RSV was used as helper to provide proteins to complement the cDNA-encoded genome analog. This analog was packaged into infectious particles and was passaged five times in culture with undiminished efficiency. This provides support for the idea that a complete, nondefective cDNA-encoded genome can be rendered biologically active so as to yield infectious virus. Work towards rescuing a complete infectious virus is described.